Light valves have been used for over fifty years for modulation of light. Early examples are described in U.S. Pat. Nos. 1,955,923, 1,962,496, 3,512,876 and 3,773,684. A conventional prior art light valve may be described as a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon usually in the form of transparent conductive coatings. The cell contains a "light valve suspension" namely small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension exhibit random Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the nature and concentration of the particles and the energy content of the light. When an electric field is applied through the light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. Light valves have been proposed for many purposes including, e.g., alphanumeric displays, television displays, windows, mirrors, eyeglasses and the like to control the amount of light passing therethrough.
For light valves of large size oriented in a vertical position, hydrostatic pressure can cause a serious problem. This pressure arises because the weight of a column of the liquid light valve suspension tends to make the walls of a light valve bulge outward, especially toward the lower part of the light valve. Methods for reducing this problem have been suggested in the prior art, e.g., see U.S. Pat. Nos. 3,742,600, 3,744,126 and 3,912,365. However, for large area light valves, such as may be desirable for use as windows, outdoor signs and other types of displays it is usually desirable that the thickness of the layer of the light valve suspension be substantially uniform over the whole active area of the light valve. If the layer thickness is not uniform, some parts of the light valve will appear darker than other parts, and application of a voltage to the device will also result in a non-uniform appearance because of non-uniform field strength from area to area. Moreover, bulging due to hydrostatic pressure can put severe strains on the seal around the cell and may break the seal.
Another problem with conventional light valves utilizing a liquid light valve suspension is the apprehension that if the cell wall or seal of the light valve is broken, the liquid light valve suspension could escape and cause damage to the user and/or objects near the light valve. This is of particular concern in eyewear applications, e.g., sunglasses, or ski goggles, where it might be believed that liquid valve suspension escaping from a broken light valve lens might harm the eye of the user.
Also, in certain applications, e.g., in eyewear applications which use for each eye a light valve comprising a sandwich of two lens blanks each coated with a conductive transparent coating (and possibly with an overcoating) with a light valve suspension therebetween, use of a suspension in liquid form would prevent one from freely shaping or machining the light valves to fit differently shaped frames.